Flow measurement device

ABSTRACT

A flow measuring device for checking on medical infusion devices comprises a vertical tube 5 having at least three level sensers 10 and 11 for sensing when a liquid reaches the level of each senser. A logic unit 12 containing a timing device triggered by the first senser measures the time taken for the liquid to reach the second senser and the logic unit compares this time with a predetermined time and if the time is less than the predetermined time sets the device to compare a time taken to reach the third senser but if the time is more than the predetermimed time calculates the flow rate. A multiway valve 1 at the inlet of the device under the control of the logic unit permits flow from the infusion device under test to the tube, from the device under test to a drain 15 or to that drain and the tube, or from the tube to the drain or blocks all flows. The logic unit can sense an excessively rapid flow and treat the flow as a dummy run to be dumped without measurement and summon a subsequent flow to be measured. The tube can be linked to the drain by a pipe 14 so that measurement can take place against a back pressure. A succession of measurements are averaged.

FIELD OF THE INVENTION

The present invention concerns a flow measurement device.

BACKGROUND OF THE INVENTION

There is a frequent need to check on infusion devices as used for avariety of treatments for infusing fluids into medical patients. Thereare a vast number of infusion devices with the different types givingdifferent rates of flow; the total range needed is from 1 milliliter perhour to 1 liter per hour and in special cases even values outside thisrange may be called for. One way of measuring flow rates is to have theinfusion device pump a liquid into a container or tube and to measurethe time difference for the liquid to pass between two sensors; knowingthe volume between the two sensors and the time difference, it is easyto calculate the flow rate. However the infusion devices do not have auniform flow rate due to sanction effects and it is difficult tocalculate flow rates over a wide range of flow rates accurately withoutchanging the tube or container. The present invention aims at providinga measuring device with a wide range of measurement without requiringcontainers or tubes to be replaced to cope with different flow rates.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a flow measurement device having a tubeor other container for receiving a flow of a liquid, a plurality ofsensors associated with the container to sense when the liquid reaches alevel in the container corresponding to each sensor, and a timing devicederiving a time difference between sensing of two levels, characterisedin that there are more than two sensers and in that logic meansassociated with the timing device checks whether the time differenceexceeds a predetermined minimum and if so calculates the flow rate fromthat time difference but if not inhibits that calculation and arrangesfor a time difference derived from a subsequent senser to be used incalculating flow rate.

By having a smaller container than usual, a quicker indication of flowrate can be given at the expense of the indication applying to aninstantaneous flow rate (which due to stiction in the infusion devicecan be erratic). The measuring device can however repeat themeasurements rapidly and the logic means derive an averaged measurementso the operator gets an initial quick indication and shortly afterwardsan averaged reading getting progressively more reliable. At the sametime the device can be smaller and more portable.

The sensors are conveniently optical sensors preferably operating in theinfra-red region of the spectrum. The tube or container can be arrangedto have increasing cross-sections as by stepping and/or tapering so asto reduce the size of the container.

The measuring device according to the present invention can be used tocheck on a wide range of types of infusion devices providing infusionrates of 1 to 1000 milliliters per hour and possibly even rates outsidethose figures without any need for changing the tube or container whichchanging might introduce calibration errors and might introduce an errordue to leakage or the introduction of air bubbles at remakeable seals.

The measuring device can be used to measure the effect of back pressureby using a suitable pressure regulator on the outlet of the device. Itis possible to arrange the device with a closed liquid system so theentire system can be pressurised with that system being connected by adrain connection to a sump.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic section of a flow measuring device accordingto the present invention.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

The flow measuring device illustrated comprises a valve 1 having aninlet connection 3 and an exhaust or drain connection 4. Upstanding fromthe valve, there is a transparent or translucent tube 5 along the sideof which in staggered relationship there is a succession of sensors 6,7, 8 and 9, the spacings between which increase progressively say in ageometric ratio of three to one. Each sensor comprises a light emittingdiode 10 and a photo-sensing cell 11. The cells' outputs are connectedto a timing and logic unit 12. The top end of the tube 5 is formed as aconverging nozzle 13 and enters a wide bore tube 14. The nozzle breaksup the size of liquid leaving the top of the tube 5 and the tube 14 hasan internal diameter large enough, say 8 millimeters, for the liquiddischarged from the nozzle not to be able to bridge that tube but to rundown the tube eliminating any risk that a mixture of air and liquid maybe sucked back into the tube 5. The tube 14 leads to a closed sump 15and is connected to the exhaust connection. The sump has associated withit a pressure regulator 16 which can set the pressure in and throughoutthe closed system of the measuring device to an above or belowatmospheric pressure. The tube 5 and the valve 1 should permit theliquid to be exhausted from the tube 5 at a faster rate than the tubefilled. It is to be noted that when the liquid is exhausted from thetube 5, the closed nature of the liquid system tends to retain theliquid so it does not enter the sump unless fresh liquid is beingintroduced into the system but lodges in a U-tube arrangement formed bythe exhaust connection and the tube 14; this U-tube arrangement shouldhave a geometry such that with the contents of the entire tube 5 in itthere is still some distance between the liquid level in the tube 14 andthe valve irrespective of back-pressure which can exceed 300 mm. Hg.This U-tube arrangement will discharge into the sump an amount equal tothe volume introduced into the tube 5 so that the levels in the U-tubearrangement stay substantially constant.

A pressure transducer 17 in the inlet connection 3 can be used with thevalve closing off the inlet connection to sense the pressure generatedby the infusion device to demonstrate the occlusion, stall and/or alarmpressure of the infusion device.

The valve 1 is operable by the logic unit 12 between conditions allowingflow from the inlet connection to the tube 5 with the drain connectionblocked, from the inlet connection to both tube 5 and drain connection,and from nowhere to nowhere with an optional further condition blockingoff the inlet connection but allowing the tube to drain to the drainconnection. Changeover between conditions should be rapid. The valve canbe of the rotary type as illustrated, of the shuttle type, of thesolenoid type or any other suitable type. The logic unit comprises atiming or clock circuit which is triggered by a first signal from a cell11. A second signal from a second cell 11 establishes a time differencewhich can be checked to ensure that it is larger than a minimumacceptable time interval and then the time difference used to calculatethe flow rate. If the signal from the second cell arrives too fast thelogic unit tells itself to look for a third signal from a third cell andto alter the volume in the flow rate calculation. If the third signal istoo fast, then the logic unit looks for a fourth signal and so on exceptthe signal from the last cell has to be used or be used to give analarm. When or as the flow rate is calculated, the logic unit energisesthe valve 1 causing the liquid to be dumped. When the first sensor or asensor especially for the purpose gives a signal denoting the liquid hasfallen sufficiently or possibly after a suitable time delay, the logicunit allows the valve to permit a second measurement to be made. Thevolumes and times involved in various readings are summed and an averageflow rate derived. The inlet tube should not be allowed to remainblocked too long or there may be a surge of liquid when flow startsagain. The problem of an opening surge if one occurs can be overcome inmany ways. The logic unit can sense an excessively rapid flow andoperate not on the first signal and a subsequent signal but on thesecond or third signals and a later signal. The logic unit on sensing anexcessively rapid flow can treat it as a dummy run not to be measuredbut to be terminated quickly so the surge is dumped with an actualmeasurement taking place quickly so there would be no or little surge onan actual measurement. However this problem is largely avoided byarranging the valve so that the contents of the tube 5 is dumped whilstthe valve permits the infusion device to continue pumping direct to thedrain connection. During a succession of measurements, the tube 5 shouldnot be allowed to empty entirely for any air in the valve could causeair bubbles which would give false readings. The presence of bubbles inthe liquid can be detected by the logic unit for if the cells claim whenthe valve is set for the tube 5 to fill to see liquid followed by noliquid this would be a clear indication that a bubble is present. Thelogic unit can then either dump the liquid or arrange for the bubble andany subsequent bubble to be pumped out through the nozzle 13.

We claim:
 1. A flow measurement device having a tube or other containerfor receiving a flow of a liquid, a plurality of sensers associated withthe container to sense when the liquid reaches a level in the containercorresponding to each senser, and a timing device deriving a timedifference between sensing of two levels, characterised in that thereare more than two sensers and in that logic means associated with thetiming device checks whether the time difference exceeds a predeterminedminimum and if so calculates the flow rate from that time difference butif not inhibits that calculation and arranges for a time differencederived from a subsequent senser to be used in calculating flow rate. 2.A flow measuring device according to claim 1 wherein a valve at an inletto be connected to the flow being measured is such as to block the flow,to direct flow to a drain connection, to direct flow to the containerwith the drain connection blocked, or to direct flow to both drainconnection and to the container, the valve being under the control ofthe logic means.
 3. A flow measuring device according to claim 2 whereinthe valve also permits flow from the container to the drain connectionwith the flow being measured blocked.
 4. A flow measuring deviceaccording to claim 3 wherein if a time difference less than a secondpredetermined minimum is sensed, the logic means operates the valve todump the contents of the container into the drain connection.
 5. A flowmeasuring device according to claim 3 wherein when the flow is into thecontainer and a senser detects a change from liquid to non-liquid, thelogic means causes the condition to be cleared before allowing ameasurement to be taken.
 6. A flow measuring device according to claim 2wherein when the logic means signals that a measurement has been taken,the logic means causes the contents of the container to be dumped andafter a delay or a signal from a bottom senser denoting that the liquidlevel has fallen below that bottom senser causes the flow to thecontainer to be restored for a subsequent measurement, variousmeasurement being averaged and displayed.
 7. A flow measuring deviceaccording to claim 2 wherein the drain connection leads to a sump whichcan be pressurised to a desired positive or negative pressure andwherein a pressure balancing tube connects the container at its top tothe drain connection.
 8. A flow measuring device according to claim 7wherein the container connects into the balancing tube through a nozzleand the balancing tube has a diameter large enough to prevent liquidbridging across it and so forming bubbles.
 9. A flow measuring deviceaccording to claim 7 wherein the balancing tube is connected to thedrain connection in such a way as to form a U-bend which has a volumesuch that there is some distance between the valve and the level in theU-bend.
 10. A flow measuring device according to claim 2 wherein apressure measuring device is provided upstream of the valve so that withthe valve blocking flow, the occlusion, stall and/or alarm pressuresgenerated by the source of the liquid can be measured.